As illustrated by U.S. Pat. No. 4,197,069 to Cloeren, an extrusion apparatus having an adjustable vane blade between flow channels, is known. The adjustable blade provides for stream convergence at substantially equal flow velocities.
As exemplified by U.S. Pat. Nos. 3,397,428 to Donald, 3,479,425 to Lefevre et al, and 3,860,372 to Newman, Jr., encapsulation of a core stream is known. Japanese Patent Document No. 55/28825 illustrates a multimanifold die that, as shown in FIG. 7 thereof, is able to produce a core layer sandwiched within an upper layer and a lower layer.
Also known is a coextrusion apparatus characterized by a removable die within a feedblock. The die is rigidly mounted between a first flow channel and a second flow channel. A core stream exiting from the die is sandwiched within streams from these flow channels.
Pivotably mounted between the first flow channel and a third flow channel of the coextrusion apparatus is an adjustable vane blade. Likewise, between the second flow channel and a fourth flow channel is another pivotably mounted, adjustable vane blade. An end of each vane blade has an undercut wall.
Each flow channel includes a tapered flow-restriction channel. As in U.S. Pat. No. 4,197,069, by adjustment of flow-restriction channel width through radial movement of a vane blade point end, the apparatus provides for the convergence of streams at substantially equal flow velocities.
Adjacent to the tip of each vane blade is a removably mounted, adjustable distribution pin. The pin is externally accessible for adjustment. Cooperation of a distribution pin with the adjacent vane blade tip, provides, if needed, for the profiling of a stream as it exits from its flow channel.
Profiling results in each layer of a laminate product having a substantially uniform widthwise cross-section. In the profiling process, the cross-sectional configuration of a stream, viewed perpendicular to the flow-direction, is transformed from a rectangular shape to a shape that is changed back to the original rectangular shape as a melt-laminate formed from converging streams, passes through a downstream die manifold.
A drawback of the coextrusion apparatus is that it does not consistently prevent lateral leakage of the core stream. As a result, a slight material degradation, observed as a discoloration with a hermally unstable polymer, sometimes occurs at the edges of a thermally unstable polymer core layer.
A further drawback is that the presence of streams on both sides of an adjustable vane blade, affords a complex flow control situation. As a result, it may be difficult for an operator to determine the necessary blade adjustment.
Furthermore, situations will arise where the streams on each side of a vane blade have opposing needs. For example, there should be a constant proportionality between the output from the first and second flow channels, and the die output. However, if, for instance, there should be a turbulent flow in the third flow channel, a vane blade adjustment to eliminatethe turbulent flow, would destroy the proportionality. Moreover, the resulting disproportionality could allow lateral leakage of the core stream.
Therefore, there is a need for an improved coextrusion apparatus that consistently prevents lateral leakage of a core stream. Such an apparatus would provide an even greater contribution to the coextrusion art if it simplified and improved flow control. Moreover, such an apparatus would make possible an improved coextrusion process for isolating a core stream.